1. Field of the Invention
The present invention relates to an injection device in a molding machine capable of efficiently heating and melting a plurality of charged pellets in a short period of time, and injecting a melting resin with superior quality.
2. Description of the Related Art
Generally, there are screw-type and plunger-type injection devices. Japanese Patent Application Laid-Open No. H6-246802 discloses a screw-type injection device typically, which is mainly constituted of a cylinder and a screw. Pellets charged from a hopper provided in the cylinder are transferred to an injection nozzle side by rotation of the screw inside of the cylinder, and are heated and melted in a transfer process. Then, the melted resin is collected to a tip-end of the nozzle, which is then injected, and the melted resin thus obtained is sent to a molding die.
A general pellet is made of resin (plastic), and a thermal conductivity thereof is about 0.07 to 0.20 kcal/m·hr·° C., which is one hundredth to one several thousandths of the thermal conductivity of metal. Therefore, it can be said that the pellet is substantially a heat insulating material. Accordingly, even if sufficient heat of fusion is imparted to the pellet for melting the pellet, heat is hardly transmitted to inside of the pellet (center portion), and much time is required for sufficiently heating the pellet.
Accordingly, it takes a long time to sufficiently melt the individual pellet in a state possible to carry out resin molding. Namely, it takes relatively a long time to melt the pellet in the cylinder, thus making work efficiency unsatisfactory. Further, in the injection device, solid matters of a plurality of pellets charged into the cylinder are heated and moved to an injection side by rotation of the screw, and at this time, a part of the plurality of pellets is set in a pressed state onto an inner wall of the cylinder.
Specifically, the pellets are partially pressed onto the inner wall of the cylinder. At this time, only a part of a surface of the solid matter of the pressed individual pellet, is brought into contact with the inner wall of the cylinder. Therefore, in the melting of the individual pellet, pellet solid matters are simply partially melted. The pellet kneaded by the screw in the cylinder is separated from the inner wall of the cylinder, thus not sufficiently heating the separated pellet. Therefore, the whole body of the solid matters of the pellets is not melted, and most of the pellets are in a state of a mixture of a melting portion and a non-melting portion.
Even in a case that the pellet is repeatedly pressed onto the inner wall of the cylinder by the screw, to thereby melt the pellet completely, and the melted pellet is transferred in the vicinity of the nozzle, an amount of the resin that stays in the cylinder is dozens of times or more of an amount required for one injection, and an excessive amount of pellet stays in the cylinder.
Further, when the melted resin passes a space between the screw and the cylinder, mechanical damage is imparted to the resin. Particularly, in a case of melting the pellet including glass fibers, lots of problems are involved, thus wearing the screw. Moreover, only a part of the pellet is melted at random, and therefore it is unavoidable that the same pellet always remains in the cylinder. Therefore, a work of exchanging materials of the pellet in the cylinder is laborious.
There is also a plunger-type in addition to the aforementioned screw-type. Such a plunger-type has a simple structure, and can be easily miniaturized. Further, the plunger-type has no defect such as wearing the screw. Japanese Examined Patent Publication No. S36-9884 discloses the plunger-type with a most basic structure, which is mainly constituted of a frustum-shaped heating cylinder having a plurality of through-holes; an injection plunger; and a supply cylinder, etc. Wherein, a synthetic resin raw material is sent to the heating cylinder by the injection plunger, thus carrying out injection. However, Japanese Examined Patent Publication No. S36-9884 also has various problems.
First, in Japanese Examined Patent Publication No. S36-9884, the injection plunger and the frustum-shaped heating cylinder are faced with each other and mutually have different diameters, and therefore the diameter of the injection plunger is formed to be slightly smaller than the diameter of an opposed part of the heating cylinder. Further, a space chamber with a wider capacity than an area of the tip-end of the injection plunger exists between the tip-end of the injection plunger and the heating cylinder, and the tip-end of the injection plunger and the supply cylinder.
Accordingly, although the melted synthetic resin raw material is extruded once into the space chamber by the injection plunger, the synthetic resin raw material cannot be efficiently flowed into the through-hole of the heating cylinder even if the injection plunger is further moved to the heating cylinder side, thus posing a problem that the synthetic resin raw material remains in the space chamber without flowing into the heating cylinder, and further posing a problem that the synthetic resin raw material remained in the space chamber becomes an obstacle to the synthetic resin raw material newly sent into the through-hole of the heating cylinder, and further posing a problem that the newly sent synthetic resin raw material and degraded resin due to long stay in the space chamber are mixed with each other.
Further, as described above, the tip-end side of the heating cylinder is formed into substantially the frustum-shape, and therefore a plurality of through-holes formed on the heating cylinder are converged toward an outflow-side from an inflow-side of the synthetic resin raw material, with an interval gradually narrowed. Namely, although the plurality of through-holes is arranged in such a manner as being properly dispersed with large intervals, outflow-side openings are crowded in a narrow range.
Therefore, the heating cylinder is formed so that intervals between through-holes are gradually narrower toward the outflow-side, thus making a cross-sectional area of a thick wall portion gradually smaller, which is the cross-sectional area of a thick wall portion excluding a formation part of each through-hole. Specifically, the heating cylinder of Japanese Examined Patent Publication No. S36-9884 is formed so that an overall heat capacity becomes smaller due to its frustum-shape, with a volume of the thick wall portion (solid portion) becoming smaller toward the outflow-side where particularly a plurality of through-holes are converged, and accordingly the heat capacity is also smaller toward the outflow-side (see FIG. 9B)
Further, according to Japanese Examined Patent Publication No. S36-9884, an inner diameter of the through-hole becomes smaller toward the outflow-side from the inflow-side, thus gradually increasing a moving speed of the synthetic resin raw material that flows and moves through the through-holes. Therefore, a heating section with small heat capacity cannot transmit a sufficient quantity of heat for heating and melting the resin, thus involving a problem that a temperature of the synthetic resin moving through the through-holes is decreased in a moving process. Therefore, in order to maintain a melting state of the synthetic resin raw material, a large heat quantity needs to be imparted to the heating cylinder, and therefore a heater band is mounted on an outer periphery of the heating cylinder.
However, as described above, the heating cylinder itself is formed into the frustum-shape, with small heat capacity and particularly with small heat capacity at the outflow-side of each through-hole, thus making it impossible to transmit a sufficient quantity of heat to the synthetic resin raw material that moves through the through-hole. Therefore, the temperature of the synthetic resin raw material is decreased, and the heat of the heating cylinder is removed, thus reducing the temperature.